Why does screen invert colors during run?

Do you have segments turned on?  Are you racing a segment?  You will get inverted screen colors if you are falling behind the time to beat in a segment. 

Side note, climb/descent is based on the ABC sensor.  Low and high pressure systems moving through your immediate area will make the watch think you're at a different altitude.  I've personally seen a really big spike from this during a track run where it thought I'd climbed almost 250 feet in a quarter mile loop, after about an hour it passed through and dropped back down.   And the same jump was also shown on my running partners activity at the same time. Pressure cells like this I think are a contributor to the "the altitude sensor sucks!' threads. you can't feel them but the watch can certainly detect them and thinks it's altitude based. 

same issue here, every time I hit a lap button or workout lap is finished on my bike activity.

Yes, I understand that strength training screens have colors based on context -- the problem is sometimes they invert from what they should be.  For example,  during a set the screen should be white and when finished with set / resting it should be black.  But that's not always the case -- and that random inversion is the issue because I react to the screen color.

Auto set is already off, and power mode is normal.  There is no rhyme or reason that I can see, but sometimes set screen is black instead of white.

Also odd -- I have background set to black.  During set, all screen black except rep screen (fine).  Finish set: rest screen black, two time screens white, HR screen black. So I have 50% of screens black and 50% are white ( at rest) -- so what color did I really select?

Also, when scrolling through screens, the ones with time data fields (white) flash to black depending on scroll direction. 

Background colors are not intuitive, particularly why some screens are white and some are black when resting.  I would expect I can set background colors for all data field screens, not just some of them.  

In any event, the real bug is that the set screen sometimes changes to white.  Others are interface choices that are not intuitive to me.

I have background set to black

Ah that must be what is causing the inconsistent behaviour for you.  I have background set to White in the Strength app and have never had an issue with it displaying white screen across all data screens when inside a set and black data screen background during the rests between sets.  But if I set the background to Black, the main (top) default Strength app data screen still sticks to Black when rest and White when inside a set, but my other user defined data screens obey the Strength app settings and are Black background when inside a set and White when resting.  I guess if you want consistent behaviour, then go in to the Strength settings and select the White background, as the unique Strength app data screens don't obey the setting change to Black.

I'm on 8.73 beta, so I'll email this in to the address on the beta page, but I doubt this will be a high priority fix as it's seemingly behaved this way for around 2 years now (I've just checked my old Fenix 5 and it's the same...).  I've also just checked the Pool Swim app and that is completely consistent; White background color setting selected equals all data screens black at rest and white when swimming, and the exact reverse when Black background color setting is selected.  So the Strength app with Black background color setting selected is seemingly the outlier here (White background color setting works just fine).